1. Field of the Invention
The present invention relates to a three-dimensional image capturing technique optimal for collectively acquiring range data as well as intensity data by means of a triangulation method and through use of a plurality of cameras, and more particularly, to an attempt to realize a reduction in measurement errors, improved usability, and a compact range finder.
2. Background Art
Techniques for measuring the geometry of an object are roughly classified into passive techniques (i.e., triangulation and shape-from-X) and active techniques (time of flight and triangulation). A difference between the passive techniques and the active techniques lies in whether or not any energy is radiated onto an object. Generally, the active techniques can be said to be more resistant to noise than the passive techniques, because ambiguity in measurement can be eliminated. The triangular technique, which belongs to both categories of active and passive techniques, is a geometrical technique which determines a range to a point of measurement located on the object on the basis of angles made between a base length and lines connecting both ends of the base length to the point of measurement. In connection with some of the active-type triangulation techniques, there has bee proposed a measurement technique for projection light of a coded stripe pattern (JP-A-3-192474). A block diagram of this technique is shown in FIG. 19. In FIG. 19, a plurality of stripe light beams encoded with colors of light by a projection system are projected onto an object, and stripe light beams originating from the object are observed by means of an image capturing system. Intensity values of the projected stripe light beams are compared with intensity values of the captured stripe light images, to thereby find the same stripe. Range data are calculated on the basis of the triangular principle.
When the object has a texture (e.g., a color or a pattern), difficulty is encountered in determining a range. Specifically, the captured stripe image is affected by the texture provided on the object and, hence, differs from the projected stripe beam in terms of color/brightness. This poses difficulty in determining which one of the projected stripe light beams corresponds to the captured stripe image. Therefore, an erroneous correspondence occurs, which in turn renders computation of a range impossible.
To solve this problem, the present inventors have conducted considerable research and eventually solved this problem by means of placing a projection system and an image capturing system at an optically-identical principal point by means of a half mirror (JP-A-2000-65542 and JP-A-2000-9442) FIG. 20 shows the configuration of this technique. Encoded stripe light is projected onto an object, and the stripe light appearing on the object is monitored through use of an image capturing system located at the same principal point as that of the projection system, and another image capturing system located at a non-principal point. The stripe image captured by the image capturing system located at the same principal point and that captured by the image capturing system located at the non-principal point include texture information about the object. Accordingly, occurrence of an error, when corresponding points are extracted by comparison between the stripe images, can be inhibited. Therefore, the influence under which measurement is deteriorated by the texture of the object can be diminished. Moreover, the texture data themselves can also be acquired accurately, whereby a three-dimensional image with a texture can be acquired.
However, when an optical system of the projection system and an optical system of the image capturing system are configured to be independent of each other as mentioned above, laborious tasks, such as those which will be mentioned below, arise when zoom ratios of the optical systems are desired to be changed in accordance with the size of an object. When a three-dimensional shape of an object is desired to be acquired with high resolution, the zoom of the optical system in the image capturing system is set to a telephoto position. Labor is required for adjusting the zoom ratio of the optical system in the projection system in accordance with the setting of the zoom of the optical system. Moreover, when the zoom ratio of the optical system in the image capturing system is desired to be changed, the position of the principal point of the optical system shifts. Therefore, very complicated tasks arise, including re-adjustment of the projection and image capturing systems, which have once been arranged in accordance with the shift, such that the optical systems of the projection and image capturing system come to an identical optical point, thereby causing a problem of deterioration of usability.
Another problem is that the characteristic of the identity of the optically principal points of the projection system and the image capturing system is realized at the half mirror, and that a strain on the half mirror influences deterioration of measurement, under the present circumstances. This is a serious problem to be solved.
Moreover, when the surface of the object has a gloss (deviation in the distribution of intensity of reflected light), a range measurement becomes difficult.
On a measurement surface having a strong characteristics of a specular finished surface, such as a measurement surface of a glossy object, specular-reflected light is observed in the direction of regular reflection. With deviation from the direction of regular reflection, the specular-reflected light diminishes. FIG. 21 is a view for describing such a reflective characteristic. The light projected from a light projection system is usually natural light, and the polarization direction of natural light is random. An image capturing system A located at the location of an eyepoint A, the eyepoint being situated in the direction of regular reflection, observes high-intensity reflected light including the specular-reflected light, as well as lambertian light. The reflected light appears in a captured image as highlight at the position of regular reflection. Therefore, the projected stripe light is observed as a stripe image having very high brightness affected by the influence of the glossy surface. Therefore, difficulty is encountered in determining which one of the projected stripe light beams corresponds to the captured stripe image. For these reasons, an erroneous correspondence arises, thereby rendering calculation of a range impossible.
Image capturing systems B, C located at the locations of eyepoints B, C, the eyepoints not being situated in the direction of regular reflection, observe reflected light including only the lambertian light. Hence, highlight does not appear in captured images. There are several cases where occurrence of erroneous correspondence is diminished by means of putting contrivance into the locations of the eyepoints as mentioned above. However, such a configuration poses a limitation on the layout of the image capturing system; that is, the configuration is incapable of adapting to measurement of a plurality of tilt measurement surfaces. Moreover, when the distribution of intensity of the specular-reflected light is not comparatively narrow differently from in FIG. 21 (i.e., a glossy object having a strong characteristic of a specular finished surface) but has a spread (i.e., a glossy object having a weak characteristic of a specular finished surface), a portion of the specular-reflected light is also observed by the image capturing system B, and calculation of a proper range is hindered by occurrence of erroneous correspondence.
FIG. 22 is a block diagram of an apparatus which attempts to eliminate the specular-reflected light by means of placing polarizing filters in front of the image capturing systems. However, even in this configuration, the reflected light having arrived at the eyepoint A situated in the direction of regular reflection is natural light having a random polarization direction. Therefore, the specular-reflected light cannot be eliminated by means of any rotational adjustment of the transmission axis of the polarizing filter. For these reasons, the aforementioned problem remains unsolved under the present circumstances.
Proposed in Japanese Patent No. 2983318 is a configuration intended for preventing deterioration of measurement when an object has a gloss as mentioned above. An illustration for explaining the configuration is shown in FIG. 23. Polarization light is generated by means of a polarized beam splitter (PBS) prism, whereupon a spot-like light beam is projected onto a glossy object. The light reflected from a glossy surface is detected by a detection section by way of the PBS prism. The light reflected from the glossy surface consists of specular-reflected light and lambertian light. The specular-reflected light is reflected by means of a characteristic of the PBS prism, and a portion of the lambertian light enters the detection section. The light having entered is detected as a point corresponding to the spot on the glossy surface. On the basis of the position detected by the detection section, the three-dimensional position of the object is calculated. If the reflected light is detected by the detection section without passing through the PBS prism, the point caused by the specular-reflected light and the point caused by the lambertian light are detected. Therefore, there arises a problem of inability to identify the three-dimensional position of the object. This configuration solves such a problem unique to the glossy object. However, this configuration involves a necessity for effecting scanning operation with projected light when the entire object is to be measured, because the projected light has a spot-like shape. For this reason, there is required an apparatus for effecting scanning operation, thereby resulting in occurrence of problems; that is, the overall measurement apparatus becoming large scale, a lot of time being consumed by scanning, poor operability, and particularly, inability to apply the object in motion.
Under the present situation, when an object is glossy, a range cannot be measured, because of the foregoing reasons, even when there is employed an optical layout characterized by the same principal point, as in the case of JP-A-2000-65542 and JP-A-2000-9442 that have been described before.